1. Field of the Invention
The present invention relates to a method for salt bath-nitriding a metal member having a blind hole, the improvement of which resides in that the interior surface of the blind hole is completely nitrided.
2. Discussion of Background
A conventional method of salt bath-nitriding a metal member having a blind hole is carried out as shown in FIGS. 3 and 4. In FIG. 3, numeral 1 indicates a metal member to be nitrided, which has a blind hole 1a. The conventional method for salt bath-nitriding is carried out by supporting a plurality of metal members 1 by a supporter (not shown) so that the open side of the blind hole 1a is directed upward and immersing them in a salt balt melt 2 in a salt bath tank (not shown) as shown in FIG. 4.
In such a method, the salt bath melt 2 does not completely invade into the bottom of the blind hole 1a and a residual air 3 often remains at the bottom of the blind hole 1a particularly when the diameter of the blind hole 1a is small.
In order to avoid this defect, there is another conventional method carried out as in FIG. 5. A plurality of metal members 1 are laid horizontally and supported by a supporter (not shown). These members are immersed in a salt bath melt 2 in a salt bath tank (not shown) to be salt bath-nitrided as shown in FIG. 6.
However, even by this method, the melt 2 does not completely replace the air in the interior of the blind hole 1a, and accordingly the residual air 3 often remains at the innermost bottom of the hole 1a when the diameter of the blind hole 1a is small.
Such formation of the residual air 3 at the bottom of the hole could not be prevented even when a through-hole with a very small diameter, for example, a perforation of a diameter of 1 mm, was provided at the bottom of the blind hole 1a of the metal member 1.
Thus, the conventional methods for salt bath-nitriding metal members as mentioned above, have a problem that air 3 remains at the bottom of the blind hole 1a and consequently the bottom part is not nitrided when the metal member 1 is immersed in the salt bath melt 2.